Clean up MetadataEnhancer

[dennisvang]

General cleanup of the MetadataEnhancer class, without changing functionality.

• switched to constructor autowiring with the help of lombok @RequiredArgsConstructor
• configured lombok to copy @Value annotations to generated constructors
• removed single-use method createMetadataIdentifier
• added javadocs for all methods, based on my interpretation of what the code actually does

This helps me understand the MetadataEnhancer internals, in preparation for taking on #854 and #605 (possibly also #858).

Assisted-by: copilot

_{AI disclosure: Copilot (free) was used to find the list of defaults for `lombok.copyableAnnotations` in the lombok source code, and for a final sanity check (no changes).}